In the case of a working machine including a working attachment having a movable distal end, an optional device may be mounted to the distal end. For example, a common hydraulic excavator is equipped with a working attachment including a raisable and lowerable boom and an arm pivotally connected to a distal end of the boom, the arm having a distal end to which a bucket is mountable. In place of the bucket, an optional device such as nipping type of crusher or breaker may be mounted to the distal end of the arm.
As a conventional system for hydraulically driving such a working attachment and an optional device, there is known one shown in FIG. 9 (Japanese Unexamined Patent Publication No. Hei 9-217385). The system includes: a first pump 91; a second pump 92; an arm cylinder 93 for actuating an arm of the working attachment; an optional cylinder (auxiliary actuator) 94 for actuating the optional device; an arm direction selector valve V1 disposed between the first pump 91 and the atm cylinder 93; and an auxiliary direction selector valve V2 disposed between the first pump 91 and the optional cylinder 94.
In this system, both the direction selector valves V1 and V2 are disposed in series along a common center bypass line joined to the first pump 91, while being connected to the first pump 91 in parallel with each other through an arm supply fluid line 95 and an auxiliary supply fluid line 96, respectively. Therefore, hydraulic fluid discharged from the first pump 91 can be supplied to the arm cylinder 93 through the arm supply fluid line 95 and the arm direction selector valve V1, and supplied to the optional cylinder 94 through the auxiliary supply fluid line 96 and the auxiliary direction selector valve V2.
However, the system involves the following problem: the load required by the optional cylinder 94 for actuating the optional device is significantly smaller than the load required by the arm cylinder 93 for actuating the arm, which generates a possibility that most of hydraulic fluid discharged from the hydraulic pump 91 is flowed into the optional cylinder 94 to thereby reduce the actuation speed of the arm cylinder 93 significantly, if no measures are taken.
To solve the problem, the system includes a variable aperture valve 97 and a control valve 98 for regulating the flow rate distribution during simultaneous driving of both of the cylinders 93 and 94. The variable throttle valve 97, which is disposed in the auxiliary supply fluid line 96, includes a pilot port and has a function of reducing the flow path area with increase in the pilot pressure supplied to the pilot port. The control valve 98, which is interposed between a pilot line of the arm direction selector valve V1 and the pilot port of the variable throttle valve 97, has a function of providing a pilot pressure to the pilot port of the variable throttle valve 97 and increasing the pilot pressure with increase in the pilot pressure input to the arm direction selector valve V1, i.e., increase in the amount of a control operation applied to an arm control lever for operating the arm.
According to this system, the flow path area of the variable throttle valve 97 is decreased with increase in the amount of the control operation applied to the arm control lever to thereby increase the flow rate of hydraulic fluid supplied to the arm cylinder 93. This makes it possible to increase a flow rate of hydraulic fluid supplied to the arm cylinder 93 with increase in the control operation amount of the arm control lever for the arm cylinder 93, even when the load required by the arm cylinder 93 is significantly greater than that required by the optional cylinder 93.
However, in the system shown in FIG. 9, although the flow rate of hydraulic fluid supplied to the arm cylinder 93 increases by the amount of reduction in the flow rate of hydraulic fluid supplied to the optional cylinder 94 at the time of combined manipulations, the flow rate of hydraulic fluid supplied to the atm cylinder 93 greatly depends on the flow rate of hydraulic fluid flowing in the optional cylinder 94. Furthermore, since the flow rate of hydraulic fluid flowing in an optional device significantly varies depending on the type or size of the optional device mounted on the working attachment, it is very difficult to stabilize the flow rate of hydraulic fluid supplied to the arm cylinder 93 so as to drive the arm cylinder 93 at a speed required by an operator regardless of the type of the optional device and the like. For example, in the case where a nipping type of crusher as the optional device is mounted to the distal end of the arm constituting the working attachment and an opening/closing control operation for opening (nipping) action or closing (releasing) action of the nipping type of crusher and a slight arm-advance control operation for raising the arm (i.e., actuating the arm in an arm pushing direction) at a restricted speed are simultaneously performed, most of hydraulic fluid discharged from the hydraulic pump is used to actuate the nipping type of crusher requiring a small load, thus preventing a driving force enough to actuate the arm in the arm pushing direction from being secured, which may disable the arm from moving.